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Helminthic Zoonoses: Echinococcosis & Trichinellosis

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PART I — ECHINOCOCCOSIS (Hydatid Disease)

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Etiology

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Epidemiology

• Over 1 million people are infected worldwide at any given time
• Highly endemic in sheep- and cattle-raising regions: Mediterranean, Middle East, Central Asia, sub-Saharan Africa, South America, parts of Australia
• E. multilocularis is endemic in central Europe, Russia, northern Japan, Alaska, and Canada
• In the U.S., most cases occur in foreign-born individuals; autochthonous cases are rare (Navajo Nation, Alaskan natives)
• Children are more commonly infected due to close contact with dogs
• Socioeconomic burden is enormous — productivity losses and medical costs estimated at $3 billion/year globally

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Pathogenesis

1. Egg ingestion — humans ingest eggs shed in feces of infected dogs/carnivores
2. Oncosphere release — gastric acid dissolves the egg shell; the hexacanth embryo (oncosphere) is released and penetrates the intestinal mucosa
3. Hematogenous/lymphatic spread — oncospheres travel via portal blood to the liver (70–80% of cases), then lungs (15%), and occasionally brain, bone, kidney, spleen, or other organs
4. Cyst formation:
   • E. granulosus → forms a unilocular hydatid cyst with three layers:
     • Pericyst: outer fibrous layer formed by host reaction
     • Laminated membrane: middle acellular layer (parasite-derived)
     • Germinal layer (endocyst): inner scolecogenic layer; produces brood capsules, protoscoleces, and daughter cysts
   • The cyst fills with clear fluid ("hydatid fluid") containing hydatid sand (free scoleces, hooklets, brood capsule fragments)
   • E. multilocularis → grows as an infiltrative, alveolar mass with no defined outer layer; invades adjacent tissues like a malignancy; can metastasize to lungs, brain

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Signs and Symptoms

• Often asymptomatic for years to decades (slow growth ~1 cm/year)
• Liver cysts: right upper quadrant (RUQ) pain, hepatomegaly, nausea, palpable mass
• Pulmonary cysts: cough, hemoptysis, dyspnea, chest pain
• Complications:
  • Cyst rupture → anaphylaxis (potentially fatal), secondary seeding (peritoneal, pleural)
  • Biliary communication → cholangitis, jaundice, biliary obstruction, cholangiohepatitis
  • Secondary bacterial infection → hepatic abscess
  • Compression of adjacent structures (IVC, bile ducts)

• Mimics hepatocellular carcinoma or cirrhosis clinically
• Presents with jaundice, hepatomegaly, RUQ pain, portal hypertension
• May metastasize to lungs and brain
• Mortality approaches 100% if untreated within 10–15 years

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Diagnosis

• Ultrasound (90–95% sensitivity): reveals cystic structure; the WHO classification (CE1–CE5) guides management:
  • CE1: unilocular simple cyst with hydatid sand
  • CE2: multivesicular with daughter cysts
  • CE3: detached floating membrane ("water lily sign")
  • CE4: heterogeneous; CE5: calcified (inactive)
• CT: better defines cyst location, number, daughter cysts, calcification, biliary involvement
• MRI: superior for biliary tract communication and brain/spinal involvement
• Pathognomonic finding: daughter cysts within the mother cyst

• ELISA (sensitivity 60–90%): screening test; higher sensitivity for liver cysts than lung/other sites
• Immunoblot (Western blot): confirmatory test; antigen 5 and antigen B (arc-5) are most specific
• Serology may be negative in up to 40% of cases (especially pulmonary or calcified/inactive cysts)

• Hydatid sand: protoscoleces, hooklets, brood capsule fragments — pathognomonic
• Histology of pericyst shows acellular laminated membrane

• Peripheral eosinophilia (variable — may be absent)
• Liver function tests may be elevated with biliary involvement

Caution: Percutaneous aspiration of intact cysts is CONTRAINDICATED without concurrent albendazole coverage due to risk of anaphylaxis and secondary seeding.

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Differential Diagnosis

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Treatment

• Albendazole 400 mg twice daily (15 mg/kg/day) in 28-day cycles with 14-day breaks
• Duration: 1–6 months depending on cyst type and response
• Always given before and after any interventional procedure (minimum 4 weeks pre-procedure)
• Mebendazole is an alternative but has poor bioavailability

• Treatment of choice for uncomplicated CE1 and CE3a cysts
• Steps: Puncture under US/CT guidance → Aspirate cyst fluid → Inject scolicidal agent (hypertonic saline 20%, ethanol 95%) → Re-aspirate
• Done under albendazole cover
• Contraindicated: cysts communicating with bile ducts, superficial/inaccessible cysts, CE4/CE5

• Indicated for: large cysts (>10 cm), complicated cysts (biliary fistula, secondary infection, extrahepatic compression), failed PAIR, CE2/CE3b
• Techniques: radical resection (pericystectomy/hepatic resection) preferred; conservative procedures (deroofing, drainage) have higher recurrence
• Intraoperative precautions: isolate field with hypertonic saline-soaked packs; avoid spillage

• Radical resection (R0) if possible — curative
• Long-term (often lifelong) albendazole if non-resectable
• Liver transplantation in selected cases (requires permanent albendazole post-transplant due to recurrence risk in immunosuppressed state)

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Prevention

• Deworm dogs regularly with praziquantel (especially in endemic areas)
• Handwashing after contact with dogs, before eating
• Avoid ingesting raw vegetables or water potentially contaminated with dog feces
• Meat inspection and proper disposal of infected animal organs
• Boiling or cooking suspected food
• Public health education in endemic regions
• Foxes (E. multilocularis): baited praziquantel traps deployed in endemic areas in Europe

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PART II — TRICHINELLOSIS (Trichinosis)

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Etiology

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Epidemiology

• 65,818 cases reported globally between 1986 and 2009 (WHO); true incidence much higher
• In the U.S.: declined from 400–500 cases/year in the 1940s to 10–20 cases/year currently — largely due to laws prohibiting feeding raw garbage to pigs
• Sources in developed countries: now primarily wild game (bear, boar, walrus, cougar) rather than commercial pork
• Outbreaks remain common in Eastern Europe, China, Southeast Asia, and South America (often linked to undercooked pork or wild game)
• Arctic outbreaks associated with T. nativa from walrus or polar bear meat
• Transmission occurs when humans eat raw or undercooked meat containing encysted larvae

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Pathogenesis

Phase 1 — Intestinal (Enteric) Phase (Days 1–7)

• Ingested encysted larvae are released by gastric acid and intestinal enzymes
• Larvae penetrate small intestinal epithelium and undergo 4 molts over 1–2 weeks → develop into adult worms
• Female adults (2.2 mm) mate and begin releasing live larvae within 5–7 days
• Adult worms cause local intestinal inflammation

Phase 2 — Migratory (Parenteral) Phase (Week 2–3)

• Newborn larvae (~100 μm) enter lymphatics/blood → disseminate hematogenously to all organs
• Larvae penetrate striated muscle fibers (preferentially those with high oxygen supply: diaphragm, masseter, tongue, intercostals, eye muscles, gastrocnemius)
• Extensive eosinophilic inflammation, vasculitis, tissue edema
• Larvae can invade heart, brain, lungs — causing myocarditis, encephalitis, pneumonitis

Phase 3 — Muscle Encystment Phase (Week 3+)

• Larvae modify host striated muscle cells into "nurse cells" (also called Nurse Cell-Parasite Complex)
• Nurse cell: a transformed myocyte that loses myofibrils, becomes granular, and develops an angiogenic network to nourish the larvae
• Larvae become encapsulated in a collagen-rich capsule; remain viable for years to decades
• Calcification occurs over time (years)
• T. pseudospiralis/papuae: do NOT form nurse cells (non-encapsulated)

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Signs and Symptoms

Phase 1 — Intestinal (Days 1–7):

• Nausea, vomiting, diarrhea, abdominal cramping
• Often mild and overlooked; may resemble gastroenteritis

Phase 2 — Invasion (Weeks 2–5):

• High fever (38–40°C) — cardinal symptom
• Periorbital/facial edema — classic finding (due to larvae invading extraocular muscles and periorbital tissue)
• Myalgia and muscle tenderness (especially masseter, tongue, eye muscles, limb muscles)
• Subungual splinter hemorrhages, conjunctival hemorrhages
• Maculopapular rash (occasionally)
• Eosinophilia — almost universal (>90% of symptomatic patients); may be dramatic (>50% of WBC)
• Elevated muscle enzymes: creatine phosphokinase (CPK), LDH, aldolase
• Respiratory symptoms: dyspnea, cough (larval migration through lungs)

Severe/Potentially Fatal Complications:

• Myocarditis: tachycardia, arrhythmias, heart failure (ECG: ST-T changes, AV block); leading cause of death
• Encephalitis/meningitis: headache, confusion, seizures, focal deficits; CSF pleocytosis (lymphocytic + eosinophilic)
• Pneumonitis: cough, hemoptysis

Phase 3 — Convalescent (Weeks 5–8+):

• Gradual resolution of fever and edema
• Persistent myalgia and fatigue may last months
• Complete recovery in most mild-moderate cases

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Diagnosis

1. Periorbital edema
2. Myalgia/muscle tenderness
3. Eosinophilia

• Eosinophilia >1,500/μL (often very high); absent in severe disease with immunosuppression
• Elevated CPK, LDH, aldolase (reflect muscle invasion)
• Normal or mildly elevated liver enzymes
• Leukocytosis common during acute phase

• ELISA (antigen from T. spiralis excretory-secretory products): becomes positive at weeks 3–4 after infection
• Western blot: confirmation
• Limitation: may be negative early in infection; false-positives can occur

• At least 1 gram of deltoid or gastrocnemius muscle
• Squash preparation (pressing muscle between glass slides) or histology
• Reveals encysted larvae within nurse cells
• Best yield at weeks 4–6
• Trichinoscopy is used for meat inspection

• Suspected meat source should be sent for trichinoscopy or digestion testing

• ECG, troponin, echocardiography for cardiac involvement

• CT brain for neurological symptoms; may show small enhancing lesions
• Chest X-ray: pulmonary infiltrates (rare)

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Differential Diagnosis

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Treatment

• Albendazole 400 mg twice daily × 10–14 days — drug of choice (better CNS penetration than mebendazole)
• Mebendazole 200–400 mg three times daily × 3 days, then 400 mg three times daily × 10 days — alternative
• Both are effective against intestinal adult worms and migrating larvae but have limited efficacy against encysted muscle larvae
• Treatment should be initiated as early as possible (during the intestinal phase)

• Prednisone 40–60 mg/day (or 1 mg/kg/day) × 5–14 days
• Indicated for: myocarditis, encephalitis, severe allergic manifestations
• Reduces larval-associated inflammation and allergic reactions
• Use simultaneously with anthelmintics (not alone — steroids alone may prolong intestinal phase)

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Prevention

• Cook pork and wild game thoroughly until internal temperature reaches ≥71°C (160°F) — until meat is no longer pink in the center
• Microwave cooking is unreliable (uneven heating)
• Well-done is the standard recommendation

• Freezing pork at −15°C for ≥3 weeks (or −23°C for ≥10 days) kills T. spiralis
• Ineffective against T. nativa (Arctic species) — freezing does NOT reliably kill it
• Smoking, drying, and curing are NOT reliable methods of killing larvae

• Ban feeding raw garbage/scraps to pigs (the most impactful public health measure in the U.S.)
• Control rodents on farms (rodents are reservoir for T. spiralis)
• Meat inspection: digestion method (pooled sample from diaphragm pillar) or trichinoscopy — mandatory in many countries

• Identify and trace the contaminated meat source
• Notify public health authorities
• Treat exposed individuals during the intestinal phase if confirmed exposure

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Summary Comparison Table

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